Preparation of a -axis YBa 2 Cu 3 O x epitaxial films using direct current-95 MHz hybrid plasma sputtering
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The dc-95 MHz hybrid plasma magnetron sputtering has been newly developed for obtaining a-axis oriented YBa 2 Cu 3 0 x (YBCO) films with an excellent crystallinity. The crystallinity was found to be the best among the films reported so far: the full width at half maximum value of 0.027° in the rocking curve measurement through the film (200) diffraction peak and ^ m i n of 2% estimated from the barium signal behind the surface peak in Rutherford backscattering (RBS) measurement using a 1 MeV He+ ion. The success in the excellent crystallinity was explained from the ion acceleration model at the ion sheath formed near the substrate surface considering the high ion density, which was revealed to be a characteristic of hybrid plasma. Almost perfect epitaxial growth was also confirmed by transmission electron microscopy. A characteristic grain boundary structure depending on the substrate was observed for the films on NdGaO3 and SrTiO3 substrates. Twist boundary is dominant for the film on NdGaO 3 , while symmetrical tilt boundary and basal-plane-faced tilt boundary exclusively exist for the film on SrTiO3. The microstructure of the film on SrTiO3 is very resistive against film relaxation. Strain relief was observed by RBS channeling spectra for the relatively high superconducting films. The results of Raman spectroscopy and RBS oxygen resonant measurements indicated that the oxygen content is not a critical parameter for determining the superconductivity of the a-axis oriented YBCO films, but oxygen ordering in the plane of the C u - 0 chain and relief of the film strain are important for the improvement of Tc.
I. INTRODUCTION Since the discovery of high Tc oxide superconductors in 1987,1 many new materials have been found in the form of bulk.2"7 Transformation of superconductors into thin films was accomplished soon after the discovery of new materials, supported by a highly developed film technology, due to the combination of strong demands in the field of semiconductors and advanced vacuum technology. Thin film technology is indispensable for designing some electric devices. In addition, film technology has an advantage in the capability of designing the crystal structure artificially irrespective of the existence of a thermodynamically stable state. For example, Schlom et al% have fabricated many bismuthbased oxide films having differently ordered layers. The technology for constructing a crystal structure artificially should be a clue for understanding the superconducting mechanism as well as for searching for new materials. However, the biggest disadvantage in thin film technology of oxide superconductors is their highly complicated structures compared with the semiconductors. The structures have been investigated in detail for some bulk oxides using x-ray diffraction (XRD) and neutron diffraction techniques, with the result that the structure and properties of YBa 2 Cu 3 0 x (YBCO) have been fairly well understood.9 In the field of thin films, YBCO has also been intensively investigated because this material 2
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